Theory

The reference pipe is what the muffler is compared with: a plain pipe from the source to the outlet.

With one inlet it is a straight cylinder with the inlet at one end and the outlet at the other.
With several inlets it is a straight trunk of this length ending in the single outlet, with one branch per inlet (Inlet branch length) forked off its start.

It is meshed and solved by the same acoustic pipeline as the muffler, at the same frequencies, with rigid walls and one uniform gas (that of the largest inlet).

With anechoic inlets the length changes only the phase of the wave in the pipe, so the power reaching a matched outlet does not depend on it. With a reflecting outlet termination, the standing-wave resonances of the pipe depend on $L$ , equation (1), and so does the reference power, peak by peak.

Equations

f_n \approx \frac{n\,c}{2\left(L + \delta\right)}

(1)

where	meaning	unit
$f_n$	Frequencies of the pipe's longitudinal resonances (with a reflecting outlet)	Hz
$n$	1, 2, 3, ...	-
$c$	Speed of sound of the pipe's gas	m/s
$L$	Reference pipe length (this setting)	m
$\delta$	End correction of the outlet termination	m

Default and rule

Default: 3 m
Rule: 3 m. Older files that stored "auto" (the inlet-to-outlet distance) load as 3 m.
Source: The length of 3 m is the program's default; it is not derived from a published rule.

Valid range > 0

Values

Above 0. The cost of the reference solve grows with the length (more elements).

Theory

Equations

Default and rule

Valid range > 0

Related 3